The present invention relates in general to devices for measuring physiological conditions such as respiration and cardiac rhythm, and more specifically, to using magnetic-field-based motion detection for such measurements.
Despite the availability of various motion sensors, measurements of physiological functions and conditions has largely been achieved by detecting electrical responses. For example, conventional heart rate monitoring is based on electrocardiograms. As another example, respiration measurement is often achieved with impedance pneumography. Another characteristic of most existing physiological measurement devices is that they require the patient to wear some sort of electrical signal sensor or electrode.
An exception is the breathing monitor disclosed in U.S. Pat. No. 5,825,293, entitled xe2x80x9cApparatus and Method for Monitoring Breathing Magneticallyxe2x80x9d, Ahmed et al. It describes a magnet placed on the patient""s chest wall, such that chest wall motion from breathing causes a changing magnetic field at a nearby magnetic field sensor. With suitable analysis, magnetic field variation data can be used to indicate whether or not the patient is breathing.
One aspect of this invention is a method of measuring mechanical activity associated with physiological motion of a living body. In one embodiment, a magnetic field sensor is placed on an area of the body. The ambient magnetic field is detected by the sensor, with the signal having time varying characteristics representative of the motion. This signal is analyzed to determine characteristics of the motion. Alternatively, a magnet (or magnetized material) rather than the magnetic field sensor may be placed on the body, and a stationary magnetic field sensor used to detect magnetic field changes. The first configuration is useful because it eliminates an additional magnet from the system, but the second may be more sensitive and it rejects external magnetic field variations.
An advantage of the invention is that the sensor system directly measures mechanical effects of motion induced by the body. There is no need to interpret electrical physiological responses.
The system operates without the need for electrical leads or electrodes. All that is required is a magnetic field sensor or magnet (or magnetized material) to be placed in the area of interest; detection is based on magnetic field variations resulting from motion induced in the magnetic field sensor or magnet (or magnetized material) located on the body.
The same sensor system can be used for a variety of applications. In one application, the magnetic field sensor is used to measure respiration. Another application is measurement of cardiac rhythms, from which conditions such as arrhythmia and heartbeat can be detected. Both the respiration and cardiac rhythm applications involve placing the magnetic field sensor (or a magnet or magnetized material), on the patient""s torso. A third application, measurement of blood pressure from vein or artery motion, is enabled by analysis of signal amplitude as well as timing. For this application, the magnetic field sensor (or a magnet or magnetized material) is typically placed on the patient""s skin over a blood vessel. A fourth application is for motion detection of internally placed catheters or leads associated with various medical devices, and uses a magnetic field sensor (or magnet or magnetized material) placed on the catheter or lead. A fifth application is motion artifact rejection, for example, to eliminate motion-induced noise in a measurement waveform. The magnetic field sensor (or magnet or magnetized material) is placed in the area of interest such that the motion-induced noise is coupled to the magnetic field sensor or magnet. This enables characterization of the noise so that it can be separated from the signal of interest.
The above-described physiological applications require the sensor system to be extremely sensitive. The invention described herein achieves this level of sensitivity. Microprocessor-based nulling techniques result in a system having low power requirements, as well as good resolution despite the presence of ambient magnetic fields, which may be large and time-varying.
A further advantage is that the sensor system can be used to simultaneously measure more than one physiological condition. For example, respiration and cardiac rhythms can be simultaneously measured by using a sensor placed on a patient""s chest or abdomen. Also, the magnet may be very small, even to the extent that they may be placed on an eyelid.